It is common practice in the petroleum industry to produce high octane motor fuel by alkylating an isoparaffin with an olefin in the presence of a catalyst which preferably is liquid hydrofluoric acid or hydrogen fluoride (HF). Such a process is commonly known as an HF alkylation process or merely an alkylation process. The effluent from the alkylation reactor containing hydrocarbons and acid, is usually passed to a generally vertically arranged settler vessel at an intermediate point along the length of the settler vessel. A hydrocarbon phase is separated from an acid phase in the settler vessel, with the hydrocarbon phase contained in the upper part of the settler vessel and the acid phase contained in the lower part of the settler vessel. Accordingly a liquid-liquid interface between the acid phase and the hydrocarbon phase is formed within the settler vessel.
In a normal operation a portion of the liquid hydrofluoric acid phase is withdrawn from the lower part of the settler vessel, cooled and then recycled to the alkylation reactor for reuse in the alkylation process. It is known in an alkylation process that there is a tendency for water to accumulate in the acid catalyst as the acid catalyst is repeatedly recycled through the system. It is also known that a material known as acid soluble oil is produced in the alkylation reaction and that this material acts as diluent for the catalyst phase. In general the production of acid soluble oil is substantially in excess of that necessary or desirable for dilution of the catalyst.
It is common practice for alkylation systems employing acid type catalyst to include a so called "rerun" system through which at least a portion of the acid catalyst is continuously passed in normal operation to remove the water and the acid soluble oil. This purification of the acid catalyst results in anhydrous acid as a product and a small quantity of an azeotrope mixture of hydrogen fluoride and water as a by-product. The by-product mixture of hydrogen fluoride and water is generally discarded because of the relative difficulty accompanying it's separation.
Because of the loss of HF acid from the rerun systems as organically combined fluoride in the azeotrope mixture of water and hydrogen fluoride, make-up acid must be added to the alkylation catalyst system either continuously or at periodic intervals. In order to have available make-up hydrofluoric acid at all times, fresh anhydrous acid is stored in an HF acid storage vessel from which fresh acid is conveyed to the alkylation process when needed. The acid may be conveyed from the HF acid storage vessel directly to the reaction zone itself or to the settler vessel.
Even in HF acid alkylation systems where make-up acid is added occasionally, for example once a month, only about ten percent of the total volume of HF acid inventory used in the alkylation process is added for make-up at any one time. Thus the volume of time HF acid storage vessel required to handle the HF acid make-up would be approximately equal to the volume of ten percent of the total HF acid inventory required for tile alkylation process.
In order to safely operate the HF alkylation process, however, an acid inventory storage vessel of sufficient volume to contain the total inventor of HF acid used in the alkylation process must be provided. This storage space is necessary to provide a safe storage vessel where the total inventory of HF acid used throughout the process could be safely stored in the event of an emergency such as a leak somewhere in the process. The total acid inventory includes acid required in the reactor, the cooler, and the settler, plus approximately 10% excess for make-up storage. Since this acid inventory storage vessel is sized so as to hold the entire HF acid inventory but, under normal operation, only contains the quantity of acid required for make-up storage, there exists a relatively large vapor space in the acid inventory storage vessel under normal operating conditions. Transfer of make-up acid from the storage vessel to the alkylation process is usually accomplished by compressed gas. This transfer generally involves pressuring the relatively large vapor space in the storage vessel with a gas such as nitrogen so that liquid HF acid is transferred from the storage vessel to the alkylation process, which in operation is at about 100 to 150 psig pressure.
The fresh anhydrous liquid HF acid required to replenish the make-up acid, or provide an initial charge of acid catalyst, is usually transported in steel railroad tank cars or truck trailers to the plant where it is being used and stored, and the acid must be transferred from the tank cars or truck trailers where the liquid HF acid is under a vapor pressure of about 35-60 psig, to the HF acid alkylation storage vessel where a pressure of about 150-175 psig is required for transfer to the alkylation process.
Transfer of HF acid from the transport vehicle to the storage vessel is usually accomplished by isolating the storage vessel from alkylation process and then venting the pressure in the storage vessel while pressuring the transport vehicle with nitrogen gas in such a manner that the liquid acid is forced through a transfer line from the transport vehicle to the acid catalyst storage vessel at a relative low pressure level compatible with the transport vehicle.
In normal operation, the relatively large gas or vapor space above the liquid HF acid level in the HF acid inventory storage vessel is therefore filled with the pressuring gas required for transferring the acid catalyst to the alkylation process. Upon venting the storage vessel, such as at the time of replenishing the make-up acid, the vent gas carries with it a considerable amount of hydrogen fluoride which is then lost from the alkylation system. This loss of hydrogen fluoride, that may have vaporized under the conditions of temperature and pressure existing in the HF acid storage vessel, when replenishing the make-up acid constitutes not only an economic loss of valuable material but also introduces a potential environmental health hazard since neutralizing of such HF containing vapors may not always be 100 percent effective. It is desired, therefore, to eliminate or at least minimize the loss of hydrogen fluoride vapor with the gases vented from a storage vessel containing liquid hydrofluoric acid.
Accordingly it is an object of this invention to improve environmental safety in operating an HF alkylation process.
A further object of this invention is to increase the safety of a petroleum refining process and the apparatus employed therein.
Yet another object of this invention is to increase the efficiency of operation of an HF alkylation process.
Yet another object of this invention is to provide apparatus and method for reducing the loss of hydrofluoric acid vapors from a vessel in which make-up HF acid is stored and must be occasionally recharged.